1. Field of the Invention
The present invention relates to a receptor layer forming composition used in a thermal transfer receiving sheet which is used together with a thermal transfer sheet having a thermal transfer ink layer, and a thermal transfer receiving sheet having a receptor layer formed by the composition. More specifically, the present invention relates to a receptor layer forming composition capable of forming a receptor layer in which a sense of surface roughness in an intermediate gradation portion is suppressed, and which has superior transfer density and image light-fastness, and a thermal transfer receiving sheet having a receptor layer formed by the composition.
2. Description of the Related Art
In the related art, a sublimation thermal transfer method is known as a technique of forming color or monochrome images. The sublimation thermal transfer method is a method in which a thermal transfer sheet containing thermal diffusion dyes having a property such that it is transferred by the heat of a thermal head or the like is disposed so as to face a receptor layer of a thermal transfer receiving sheet, and the thermal head is pressed against the rear side of the thermal transfer sheet, thus heating the thermal transfer sheet. The sublimation thermal transfer method forms images by heating the thermal transfer sheet so that dyes are transferred to the receptor layer as the images. Such a sublimation thermal transfer method has been noted as a method which enables image formation using digital data and gradation expressions comparable to silver halide photography without using a treatment solution such as a developing solution.
The receptor layer of the thermal transfer receiving sheet is generally formed using various thermoplastic resins as its main component. Examples of such thermoplastic resins include polyester resins, polyamide resins, acrylic resins, acetate resins, polyvinyl chloride-polyvinyl acetate resins, styrene resins, polyvinyl butyral resins, cellulose resins, and polycarbonate resins. For example, Japanese Unexamined Patent Application Publication No. 57-107885 proposes a receptor layer which is provided on a base material as a coating layer containing saturated polyester, polyvinylpyrrolidone, and pigments. Japanese Unexamined Patent Application Publication No. 60-122192 proposes a receptor layer which is formed from at least one resin selected from polyester resins, polyamide resins, acrylic resins, and acetate resins and at least one resin selected from hydrocarbon-based resins and fluorine-based resins or silicon resins.
These proposals are made to improve transfer density and image light-fastness, neither of which provides satisfactory results. Particularly, images can easily fade with light, and high-resolution printing images may not last over a long period. The reasons for this are associated to the recent progress in the printing speed of thermal transfer printers. That is, in the receptor layers as proposed hitherto, even when they are used in high-speed thermal transfer printers, since the printing time is short, it is difficult to diffuse dyes to be fixed to a receptor layer. Therefore, in the existing receptor layers, dyes are present in a state of being aggregated near the surface of a receptor layer. Thus, transfer density and image light-fastness are deteriorated.
As a base material of a thermal transfer receiving sheet, for example, Japanese Unexamined Patent Application Publication No. 2-592 proposes a synthetic paper as available under the name YUPO (trademark). Moreover, Japanese Unexamined Patent Application Publication No. 3-268998 proposes a laminate in which a foamed polyolefin layer is laminated on both surfaces of a core material.
However, a thermal transfer receiving sheet using such a base material is sensually different in finger-touch feeling, stiffness or texture, subtle difference in surface state or the like as compared to silver halide photographic paper. Thus, the thermal transfer receiving sheet is not satisfactory as a substitute for silver halide photographic paper. As a base material, for example, Japanese Unexamined Patent Application Publication No. 2-106397 or Japanese Unexamined Patent Application Publication No. 4-115992 proposes that a polyolefin laminated paper sheet in which a polyolefin layer is provided on the surface of a paper containing natural pulp as a main component may be preferably used.
However, the use of the polyolefin laminated paper sheet only is not sufficient for realizing surface smoothness, cushion, and heat insulation as necessary for the base material of the thermal transfer receiving sheet. As a base material of a thermal transfer receiving sheet, for example, Japanese Unexamined Patent Application Publication No. 2008-6748 and Japanese Unexamined Patent Application Publication No. 2008-6786 propose a base material which is coated with a layer containing hollow particles or foaming fine particles and an intermediate layer. The use of such a base material enables improvement to the coloring density and color reproduction properties while providing the same touch feeling and stiffness as the silver halide photographic paper.
However, the base material in which an intermediate layer or the like is provided so as to improve the surface smoothness, cushion, or heat insulation as proposed in the patent documents is not comparable to a base material to which a synthetic film or a synthetic paper is bonded. Therefore, it is difficult to obtain satisfactory properties such as transfer density even when the existing receptor layer is provided on the intermediate layer. Japanese Unexamined Patent Application Publication No. 10-337967 proposes a receptor layer using a core/shell structure polymer in which the core and shell each have a predetermined glass transition temperature Tg. Japanese Unexamined Patent Application Publication No. 10-309874 proposes a receptor layer having a polymer which has a core/shell structure composed of a urethane resin and a vinyl-based polymer or a micro domain structure. When a receptor layer forming composition for thermal transfer recording using a substantially water-dispersible emulsion was provided on an intermediate layer of a polyolefin laminated paper sheet used for silver halide photography, the sense of surface roughness was particularly strong in an intermediate gradation portion.